1. Field of the Invention
The invention relates to an arrangement and method for processing material with a laser beam that is guided by a liquid jet.
2. Description of the Prior Art
Laser irradiation is used in a variety of ways for the material processing in the industry--cutting, drilling, welding, marking and material stripping. Nearly all types of material, e.g. steel, steel alloys, nonferrous metals, plastics and ceramics can be processed.
For nearly all of these methods, the laser beam is focused with the aid of an optical element, for example a lens, on the material to be processed in order to generate the intensity necessary for the processing operation. Due to this necessary beam focusing, a processing was possible only at the location of the focus point or the area immediately surrounding it.
From reference DE-A 36 43 284, a method for cutting a material with a laser beam is known, for which this beam was coupled with a water jet that was directed toward the material to be cut and was guided within this water jet. The feeding of the beam is by way of a beam guide (fiber), one end of which projected into the water jet that was produced in a nozzle. The diameter of the water jet was greater than that of the beam guide. The known arrangement had the disadvantage that the water jet diameter could never be smaller than that of the beam guide.
However, in order to achieve a high intensity at the processing location, a smallest possible beam diameter is necessary. The smaller the beam diameter, the lower the output of the laser radiation source with which it is possible to work. The smaller the output capacity of the laser, the lower the purchasing price.
Another disadvantage of the arrangement according to DE-A 36 43 284 resulted from the beam guide end that projected into the water jet. A dead water zone formed below the guide end which, among other things, produced interruptions in the flow that increased exponentially over the length of the water jet and finally resulted in a separation of the jet into drops. That is why it was impossible with this arrangement to achieve a laminar, compact beam length of more than 30 mm.
In the reference EP-A 0 515 983, an attempt was made to eliminate the above disadvantages by designing a water nozzle, which no longer contained the beam guide directly. In front of the nozzle that forms the water jet, there was a water chamber with a water intake and a focusing lens that closed off the chamber to the nozzle intake. This focusing lens is a component of an optical system, with which it was possible to focus the radiation emitted by the beam guide into the nozzle duct of the nozzle. The chamber was designed such that the water inside for the water jet was kept in a quasi standstill condition, that is in a relaxed condition.
However, it has proven that this second design variant of a laser beam to be coupled with a water jet causes uncontrollable damages to the nozzle wall in the area surrounding the nozzle duct intake.
It is the object of the invention to create an arrangement with which a laser beam can be coupled optimally with a fluid jet for material processing, without damage being caused to the nozzle that produces the fluid jet by the laser radiation.
The invention is based on the realization that the laser beam that is focused with the focusing optics into the nozzle range can heat up the liquid more or less strongly, depending on the intensity distribution. Liquid ranges with varied temperatures, a spatial temperature gradient, not only have a spatially specific density distribution, but also a spatial refractive index distribution. Meaning liquids with a spatial temperature gradient react optically as a lens and, as a rule, in the focusing cone of a focused laser beam as a dispersing lens.
An "optimally designed" coupling of the laser radiation with a liquid (water) jet produced in a nozzle duct, as shown in FIG. 2 of the EP-A 0 515 983, unfortunately does not work as assumed. For the arrangement as shown in the EP-A 0 515 983, a thermal lens formed in the range of the focusing cone point, above the nozzle duct intake, which moved the location of the focusing point shown there upwards, as well as increased the focusing diameter considerably. As a result of this, a portion of the laser radiation in the focusing cone hit the nozzle wall, in particular the nozzle surface that faces the liquid retaining chamber used there. As a result of the high intensity which is necessary for the material processing, the wall of the nozzle was consequently damaged.
With the design according to the EP-A 0 515 983, it was of disadvantage that the liquid used was water and the laser radiation was from a ND:YAG with 1.064 .mu.m. This radiation exhibited especially in water an absorption quality that could not be ignored. The water in the range of the upper pyramid tip of the focused radiation (point range of the focusing cone) was heated according to the intensity distribution (high intensity in the axis and low intensity at the edges) and the above predicted thermal lens developed, which led to damage to the nozzle wall, in particular the nozzle surface in the range of the nozzle intake and in the final analysis led to the destruction of the nozzle that produces the liquid jet.
It was not only the use of water that decreased the degree of effectiveness of the coupling, but also the total structural design of the liquid chamber in front of the nozzle intake. Attempts were made, as is explained on page 7, lines 32 ff and page 9, lines 41 ff, to achieve if possible a standstill in front of the nozzle intake. However, it is this standstill of the liquid that made possible or furthered the formation of the thermal lens. The liquid heated by (even by only a slight) radiation absorption was not removed as fast as possible so that it could not heat up any more, which would have reduced the lens formation effect. On the contrary, this increased heating up caused an even higher refraction for a thermal lens that formed.
However, the invention takes a different route. In this case, everything is done to prevent if possible the formation of a thermal lens or to strongly minimize its effect. The invention uses a liquid with the smallest possible absorption of the laser radiation used, namely silicone oil if the radiation is from the ND-YAG laser.
Furthermore, the structural design of the processing module that includes the nozzle arrangement and the focusing unit is selected such that even for a small radiation absorption that cannot be neglected, the effect of a thermal lens, insofar as it should occur at all, is at a minimum and thus can be neglected.
The invention suggests that the liquid from the range of the focusing cone for the laser radiation, in particular from the area surrounding the point, be removed as fast as possible to keep the heating up time as short as possible. The best results are of course achieved if the liquid remains only for a short time in the focusing cone with a low absorption.
In order to achieve the above conditions, the EP-A 0 515 983 with the liquid retaining chamber used there to hold the liquid at a standstill, which is propagated there, is avoided completely. The level of liquid feeding to the nozzle has approximately the diameter of the nozzle duct or is only slightly larger, simply to reduce the swirling of the flow.
Also, for a preferred design version, no focusing lens is installed in the wall opposite the nozzle intake, as is the case with EP-A 0 515 983, but only a window, which transmits the laser radiation without losses. Only this window, which is located almost directly above the nozzle intake, makes it possible to keep the liquid volume in the point of the focusing cone as small as possible and the flow speed as high as possible.
In another preferred embodiment, the fluid intake edge for the nozzle duct has a sharp edge design. As a result of this sharp edge, a liquid jet separation occurs at the intake edge with an air bolster between the liquid jet and the nozzle duct wall. Air has a smaller refraction index than the generally used nozzle materials such as quartz or sapphire. Also, the refraction index of air is smaller than that of the liquid to be used, meaning a liquid jet forms that functions as nearly ideal radiation guide. The laser radiation guided inside the liquid jet is thus "insulated" against the nozzle wall.
The air bolster proves itself in particular if the refraction index of the liquid jet guiding the radiation is smaller than the one for the nozzle because an outflowing of the radiation could then take place. If the refraction index for the liquid is bigger than the one for the nozzle material, then a total reflection occurs at the interface between the two mediums (liquid/nozzle wall), but the radiation enters the other material until it reaches the so-called penetration depth ("frustrated total internal reflection", "Goos-Hanchen-Shift"). If the nozzle material has an absorption coefficient that can no longer be neglected with the radiation used, then the air bolster here prevents a penetration of the radiation as well and thus also a damaging of the nozzle walls.
Thanks to the design of the air bolster, even materials that absorb the laser radiation can be used for the nozzle because the bolster keeps these away from the nozzle walls. Furthermore, an adjustment of the focusing cone to the nozzle duct axis is no longer absolutely necessary as a result of this because the radiation can no longer reach the nozzle duct walls due to the air bolster, even with a slight maladjustment. The angle for the focusing cone now can be selected such that it corresponds to the theoretical value for the numerical aperture of the liquid jet.
If, for example, the liquid is selected to be an oil from the group of silicone oils, e.g. a polydimethylsiloxane or a polymethylphenylsiloxane and quartz as nozzle material, then the refraction index of the liquid is higher than that of the nozzle material. The nozzle duct then has the quality of a beam guide, even if there is no air bolster. In that case, the length and shape of the nozzle duct is not critical for the beam guidance. It would even be possible to realize very long and bent nozzle ducts.
Of course it is possible to use other electrically conducting as well as in particular electrically non-conducting liquids in place of silicone oil. They must be selected such that their absorption for the laser wave length used remains within an acceptable frame in view of the flow speeds than can be achieved in the feed lines in the area of the nozzle in order to avoid a thermal lens. We want to point here especially to those types of liquids, which are used for liquid fibers as usable liquids.
The suggested arrangement permits liquid jet lengths of more than 200 mm. If an uninterrupted flow is ensured at the intake of the nozzle duct, the liquid pressure can be increased and the compact liquid jet length increased to a maximum, which depends above all on the liquid used and the nozzle diameter. Thus, for example, it follows that for water and a nozzle duct diameter of 150 .mu.m, a maximum compact jet length of 150 mm for 80 bar liquid pressure results. If a silicone oil is used in place of water, then the compact liquid jet length can be increased to up to 500 mm. The length prior to the start of the "disintegration into drops" is referred to as compact liquid jet length. This disintegration into drops is based on unavoidable swirling, caused by the surrounding air as well as the surface tension.
The disintegration length of the liquid jet can be varied by way of the pressure of the liquid before it enters the nozzle duct. However, causing a purposeful interference to the liquid immediately prior to the nozzle duct intake is more elegant. This can be done, for example, with a piezo element, which exerts pressure jolts of a predetermined frequency and amplitude onto the liquid. The length of the liquid jet then depends on these parameters. The adjustment of the liquid jet length is important particularly if the layers below the material to be processed must not be hit by the laser beam.
Furthermore, the absorption of the above-named oil over a wide wave-length range of the radiation is lower than that of water, so that on the one hand the work length is no longer limited by the absorption in the liquid, and on the other hand, the effect of the thermal lens in front of the nozzle is avoided or strongly reduced. At the same time, this also provides a protective effect against corrosion during and after the processing of the work piece.
Silicone oils have a series of advantageous qualities for this type of material processing. Namely they have an excellent resistance to oxidation, hydrolysis and weathering. Also, they exhibit a chemical indifference that excludes a corrosion danger. Furthermore, they distinguish themselves by an extremely low combustibility as well as a high compressibility.
As a result of the long length of a liquid jet with nearly constant, high radiation intensity, a working length that is increased several times is obtained. This makes it possible, among other things, to process multilayer objects, e.g. objects consisting of two glass sheets with an air gap . . . , because the liquid jet conducting the high-energy laser radiation, which emerges from a cut joint or a hole for the most part retains its quality as beam guide.
An optimum coupling of the laser radiation is achieved if the focal point is placed in the plane for the nozzle opening. The underside for the laser radiation transmitting window, which faces the nozzle opening, should be at a distance of 200 .mu.m to 500 .mu.m for a nozzle diameter of 100 .mu.m. This avoids a liquid retaining chamber that would favor the formation of a thermal lens.
In addition, the laser beam conducted in the liquid jet, permits parallel cut edges. This makes it possible to process larger material strengths, among other things with a lower material loss.
By coupling the laser beam with the liquid jet, the beam quality of the laser, which among other things is made worse by the beam guidance in a beam guide, plays a subordinate role. This lowers the purchasing costs for the laser. A beam guide for guiding the beam from the laser source to a coupling place within the working module also solves all safety problems for the beam guidance by the user. For the conventional beam focusing with only one focusing system without liquid jet, the poorer beam quality would lead to an even shorter working length.
The use of a beam guide, starting with the laser source, the small geometric dimensions of the coupling arrangement and the precise control that is no longer needed for a working distance between focusing unit and the surface of the work piece to be processed result in a simple displacement or sliding arrangement for the processing module.
The danger arising with the conventional beam focusing of having dirt on the focusing lens or a glass that protects this lens, caused by the spraying of material that is stripped from the work piece, are nonexistent here. Furthermore, the liquidity of the liquid jet ensures a very efficient cooling of the processing zone, so that no thermal stressing of the work piece along its processing edges can occur. For example, it is no longer possible for the material to pull out of shape when cutting small webs. In addition, this cooling leads only to a very slight hardening of the processing zone, which makes a reworking, e.g. an additional thread cutting, very simple.
A gas and dust development is avoided at the same time because the liquid bonds the stripped material. This eliminates the need for expensive exhaust air filtering systems.
Furthermore, melting occurrences are avoided or greatly reduced by the impacting liquid. The processing quality is very high. Also, it is possible to achieve protection against corrosion for corroding materials through a suitable selection of the liquid.
In a comparison with the known water jet cutting of work pieces, for which a water jet with mixed-in abrasive materials is used under considerably higher pressure relative to the above method according to the invention, a simple hydraulic system with flexible hydraulic lines can be used with the low pressure necessary for this method. This results in a simple slider system for the processing module. Furthermore, the strong wear of the nozzles, as it occurs with the water jet cutting, does not exist. Also, the slowing down of the liquid jet does not present any problems with the method according to the invention.
Water is the only liquid known from prior art for the liquid jet. Liquids other than water could be used only after liquid forming the liquid jet was captured, reprocessed and reused in particular in a closed circulation, for economic reasons.
After designing the liquid feed line or feed lines without liquid retaining chambers in the focusing channel tip area coordinated with the nozzle duct, it was determined that insofar as the nozzle material and also the liquid consist of electrically insulating material, an electrical charging of the liquid for the jet takes place. The charging results in voltages of more than 5 kV. If this electrically charged liquid jet is directed toward the material to be cut, it discharges its charge to this material. If, for example, copper or aluminum is processed with the radiation of an Nd:YAG laser coupled with a liquid jet, it can be determined that the material stripping rate strongly depends on the charge of the jet.
For a pulsed Nd:YAG laser radiation with 250 mJ, a pulse width of 0.1 ms and a liquid pressure of 10 bar, there is only a very slight material ablation. If the liquid pressure is increased to 100 bar, then an excellent stripping rate is achieved. This liquid jet with coupled-in laser radiation then produces a plasma on the material to be processed at a much faster rate because of its electrical charge, which increases the stripping rate. This effect can be increased even further by increasing the pressure to, for example, 1000 bar or by a directed electrical charging of the liquid in front of the nozzle intake.
Furthermore, the electrical charge of the liquid jet can be used in that its deflection is caused by a neighboring electrical field.
In the following, examples of the device according to the invention are explained in more detail with the aid of drawings. Advantages of the invention follow from the subsequent descriptive text.